At present, three methods are mainly used for synthesis of carbon nanotube fiber: wet spinning, carbon nanotube array direct spinning method and floating chemical vapor deposition drawing method. The pure double-wall carbon nanotube fiber made by Rice University [NatnaelBehabtu et al. Strong, Light, Multifunctional Fibers of Carbon Nanotubes with Ultrahigh Conductivity. Science 339, 182 (2013)] of USA with the wet spinning method shows excellent mechanical properties and electrical properties. However, it is difficult to use the wet spinning method widely since the cost of the method is too high (market price: USD 2,000/g). After copper doping, the conductivity of the carbon nanotube fiber, which is made by Hata research group [Chandramouli Subramaniam et al. One hundred fold increase in current carrying capacity in a carbon nanotube-copper composite. DOI: 10.1038/ncomms3202.] of Japan with the carbon nanotube array direct spinning method, can approach to that of copper, and its carrier density is about 100 times of that of copper. As the large scale of grown carbon nanotube array is limited by the size of silicon substrate, the array spinning method is unsuitable for industrialized production. A plurality of carbon nanotube fibers with a length of several kilometers, which is made by Tianjin University [Xiao Hua Zhong et al. Continuous Multilayered Carbon Nanotube Yarns. Adv. Mater. 2010, 22, 692-696] with the floating chemical vapor deposition drawing method, show good mechanical properties. However, the electrical properties of the carbon nanotube fiber synthesized with this method are poor mainly due to the synthesis process of making carbon nanotube with floating vapor deposition method at the current stage. At present, the number of walls of carbon nanotube synthesized with this method is uneven, and the carbon nanotube contains a plurality of carbon impurities and catalyst particles as it is drawn directly without purification, thus influencing the electrical properties and mechanical properties of the carbon nanotube fiber.